A tank ventilation system consists of a fuel tank, an adsorption filter, and a ventilation line connecting the fuel tank to the adsorption filter. Furthermore, the adsorption filter is connected to the atmosphere by way of a fresh air line in which a controllable check valve is located as the adsorption filter check valve (AAV). Moreover the adsorption filter is connected to the intake manifold of the internal combustion engine by a regeneration line in which there is a controllable regeneration valve as the tank ventilation valve (TEV).
In operation of the internal combustion engine with the check valve (AAV) opened and the tank ventilation valve (TEV) opened, the fuel vapors outgassing from the fuel are intaken by the negative pressure prevailing in the intake manifold. Furthermore, the fuel vapors stored temporarily in the adsorption filter are also disposed of and thus the adsorption filter is regenerated. With the vehicle stopped the tank system is ventilated by way of the adsorption filter, the outgassing fuel vapors being temporarily stored therein.
For reasons of environmental protection, measurement methods are being increasingly required by means of which leaks in tank ventilation systems of motor vehicles are detected and displayed.
Such a generic method is known from DE 40 03 751 A1. In this connection the adsorption filter check valve (AAV) is closed and the tank ventilation valve (TEV) is opened, the system is evacuated and after closing the TEV by means of a pressure sensor it is checked whether the negative pressure decay gradient in the tank ventilation system corresponds to a boundary negative pressure gradient. This method has the disadvantage that the negative pressure generated by way of the intake manifold negative pressure in the tank ventilation system is not constant, but depends on variable parameters. This method is not accurate enough for detecting smaller leaks. In particular a small leak cannot be easily detected by such a simple pressure test, since the fuel based on its vapor pressure behavior builds up a natural pressure depending on the temperature and outside pressure and other effects such as the fill level, fuel quality, or mechanical movements of the fuel tank can mask the effect of a small leak.
Developments of the aforementioned method to be able to detect in particular also smaller leaks are known, in particular a correction being carried out for the outgassing of fuel. For example, in DE 42 27 698 C2 control is exercised to a specified negative pressure with a stable air-fuel mixture, and proceeding from there the pressure variation in the blocked tank ventilation system is detected and evaluated for a tightness diagnosis. In another known method as claimed in DE 197 13 085 A1, the influence parameters are detected and complex correction computations are carried out based on a physical model for the measured pressure variation. In another known method as claimed in DE 44 27 688 C2 the dynamic behavior of the pressure variation is detected using several successive pressure values and an average value formed therefrom is evaluated. in DE 101 43 329 A1 correction values are determined by evaluating the pressure changes in several measurement cycles.
The object of the invention is to develop a generic method for checking gastightness of a motor vehicle tank ventilation system such that with a simple structure of the tank ventilation system and simple process guidance the gastightness of a tank ventilation system can be reliably assessed.